JP5839180B2 - Method for cooling hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for cooling a hot-rolled steel sheet suitable for producing an original sheet of a hot-dip galvanized steel sheet.

  As shown in FIG. 1, a hot-rolled steel sheet, which is one of the hot steel sheets, is rolled into a rough bar by rolling a slab heated to a predetermined temperature in a heating furnace 1 with a roughing mill 2, and then the rough bar. Is rolled to a predetermined thickness in a continuous hot finishing rolling mill 3 composed of a plurality of rolling stands, and further, cooling water is supplied to the upper and lower surfaces by a cooling device 4 installed on the run-out table. In general, after cooling to a predetermined temperature, the coil is wound by a winder 5 to produce a coil.

  An alloyed hot-dip galvanized steel sheet having high workability is manufactured through the processes of pickling, cold rolling, annealing, and plating using the hot-rolled steel sheet that has undergone the above steps.

  By the way, in the case of a hot-rolled steel sheet containing an alloy element such as Si or Mn, the oxide of Si or Mn is concentrated on the surface of the steel sheet in the annealing process before the plating process. In some cases, the adhesiveness (hereinafter referred to as “plating property”) was deteriorated.

  For this reason, conventionally, as disclosed in Patent Document 1, during the annealing, the dew point and the like are controlled, and the oxidation of Si or the like is concentrated on the surface of the steel sheet to suppress the deterioration of the plating property. .

  On the other hand, in Non-Patent Document 1, an oxide layer of an easily oxidizable element such as Si (hereinafter referred to as an internal oxide layer) generated inside the iron oxide generated on the surface layer after winding of the steel sheet is appropriately used. It has also been reported that plating properties can be improved by forming under conditions.

JP 2009-209397 A

Surface Technology Association Surface Technology Research Paper Vol. 55, no. 1,2004 n48-55

  The production amount of the internal oxide layer (hereinafter referred to as internal oxidation amount) is determined by the temperature history of the hot-rolled steel sheet wound in a coil shape. For example, as shown in FIG. It has been shown that the layer formation rate is fast at 600 ° C. and above and very slow at 450 ° C. and below.

  Usually, the surface rate of the coil, such as the end portion in the plate width direction of the coil, the inner peripheral portion, and the outer peripheral portion, has a higher cooling rate than the inner portion of the coil.

  Therefore, when the temperature of the coil surface part is equal to or lower than that of the other part when winding the hot-rolled steel sheet, the amount of internal oxidation is less than that of the other part, and the plating property is higher than that of the other part. Getting worse.

  Moreover, if the amount of internal oxidation varies significantly in each part of the hot-rolled steel sheet, even when the method disclosed in Non-Patent Document 1 is applied, plating defects may occur locally.

  In particular, the internal oxide layer of Si is generated during the cooling after the hot rolling is finished and wound into a coil.

  The hot-rolled steel sheet before the pickling treatment is an iron oxide layer, an Si internal oxide layer, an Si deficient layer, and a ground iron in order from the surface. During the pickling treatment, the iron oxide layer and the Si interior The oxide layer is removed, and the hot-rolled steel sheet is then cold-rolled and annealed.

  In the annealing treatment, a reducing atmosphere is formed for iron, but an oxidizing atmosphere is formed for Si, which is an easily oxidizable element. Therefore, when the amount of Si in the steel is large, Si diffuses to the surface of the steel sheet and is oxidized on the surface of the steel sheet. Things are easy to form.

  In addition, Si oxide has poor wettability with zinc, and if Si oxide is present on the surface of the steel sheet, the plating property deteriorates. Therefore, it is particularly important to prevent the surface oxidation of Si during the plating process. It becomes.

  On the other hand, the thicker the Si-depleted layer, the more the surface oxidation of Si is suppressed, so that the plating property is improved. However, when the amount of internal oxidation of Si is small, the Si-depleted layer existing inside is also thinner. As a result, the plating property is deteriorated.

  In addition, when the amount of internal oxidation of Si is large, the internal oxidation layer of Si cannot be removed in the pickling treatment, and the plating property deteriorates. Therefore, in the pickling treatment, the internal oxidation amount of Si must be controlled within an appropriate range.

  Then, the objective of this invention exists in the place which proposes the cooling method for equalizing the internal oxidation amount of a hot-rolled steel plate in the suitable range over the longitudinal direction and the width direction in view of said situation.

  The present invention introduces a hot-rolled steel sheet having a Si content of 0.2 mass% or more into a cooling facility disposed downstream of the hot rolling line, and supplies cooling water to the upper and lower surfaces of the hot-rolled steel sheet. In the cooling method, it corresponds to a region of 50 to 150 mm from the width end of the hot-rolled steel plate toward the center thereof, and a length of 5 to 30% along the longitudinal direction of the tip and tail ends of the hot-rolled steel plate. A method for cooling a hot-rolled steel sheet, characterized in that at least one of the regions is maintained at a temperature higher than the temperature of the steady portion.

ｔ ：時間（秒）
ｔｃ：巻取り開始時間（秒）
ｔｅ：常温に至るまでの時間（秒）
Ｔ ：絶対温度（Ｋ）
ａ ：８〜９×１０^−６
ｂ ：１.５〜２.５×１０^４
が、本発明の課題を解決するための具体的手段として好ましい。 In the method for cooling a hot-rolled steel sheet having the above structure,
1) The hot rolled steel sheet has a winding target temperature of 450 to 600 ° C.
2) The temperature of the region corresponding to a length of 50 to 150 mm from the width end of the hot-rolled steel plate toward the center thereof and the length of the tip portion and the tail end portion of the hot-rolled steel plate is 5 to 30%. 50 to 100 ° C. higher than the stationary part,
3) The hot-rolled steel sheet is such that the thickness (δ) of the internal oxide layer of Si at room temperature after winding satisfies the following conditions:
Record
1.0 × 10 ⁻⁶ ≦ δ ≦ 4.0 × 10 ⁻⁶ (1)

t: Time (seconds)
tc: Winding start time (seconds)
te: Time to room temperature (seconds)
T: Absolute temperature (K)
a: 8-9 × 10 ⁻⁶
b: 1.5-2.5 × 10 ⁴
However, it is preferable as a specific means for solving the problems of the present invention.

  According to the present invention having the above-described configuration, a hot rolled steel sheet having a Si content of 0.2 mass% or more is introduced into a cooling facility disposed downstream of the hot rolling line, and the upper and lower surfaces of the steel sheet. When cooling water is supplied and cooled, the region of 50 to 150 mm from the width end of the hot-rolled steel sheet toward the center thereof, and the longitudinal direction of at least one of the front end portion and tail end portion of the hot-rolled steel plate 5 Since at least one of the regions corresponding to the length of ˜30% is maintained at a temperature higher than the temperature of the steady portion, the amount of internal oxidation of Si becomes uniform in the longitudinal direction and the width direction of the steel plate, and the steel plate The overall plating property of is improved.

  Moreover, according to the cooling method of this invention, it is 5-30% along the longitudinal direction of the area | region of 50-150 mm which goes to the center part from the width end of a hot-rolled steel plate, and the front-end | tip part of a hot-rolled steel plate, and a tail end part. Since at least one of the regions corresponding to the length is set to a temperature 50 to 100 ° C. higher than that of the stationary part, the internal oxidation amount of the part becomes equal to the internal oxidation amount of the stationary part.

Furthermore, according to the cooling method of the present invention having the above-described configuration, the hot-rolled steel sheet has a thickness (δ) of the Si oxide layer after cooling at room temperature of 1.0 × 10 ⁻⁶ ≦ Since it falls within the range of δ ≦ 4.0 × 10 ⁻⁶ , an Si-deficient layer having an appropriate thickness can be formed, and an internal oxide layer of Si can be removed in the pickling process. Contributes to improvement of sex.

FIG. 1 is a diagram schematically showing a hot rolling line. FIG. 2 is a graph showing the relationship between the generation rate of the internal oxide layer and the temperature. FIG. 3 is a diagram showing a change in temperature in the width direction of the hot steel sheet. FIG. 4 is a diagram showing the relationship between temperature and time in the width direction of the hot-rolled steel sheet. FIG. 5 is a diagram showing a change in temperature in the longitudinal direction of the hot-rolled steel sheet. FIG. 6 is a diagram showing the relationship between temperature and time in the longitudinal direction of the hot steel sheet after hot rolling.

  The cooling rate in the coil inner peripheral part, outer peripheral part, and width direction edge part of the hot-rolled steel sheet wound up in a coil shape is faster than that in the coil. Therefore, the amount of internal oxidation at the site is reduced, and the platability is likely to deteriorate.

  Further, in a hot-rolled steel sheet having a Si content of less than 0.2 mass%, the plating property is not deteriorated even if the internal oxidation amount is somewhat uneven, but the Si content is 0.2 mass%. In the above-described hot-rolled steel sheet, if the internal oxidation amount is not uniform, the plateability is deteriorated.

  Therefore, it is necessary to perform appropriate temperature control for each part of the coil and to uniformize the amount of internal oxidation within an appropriate range over the entire length and width of the steel sheet.

  In this invention, it is preferable that the winding target temperature of a hot-rolled steel plate shall be 450-600 degreeC. This is because when the coiling temperature of the hot-rolled steel sheet exceeds 600 ° C., as shown in FIG. 2 above, the generation rate of the internal oxide layer becomes very fast, and the amount of internal oxidation becomes excessive. On the other hand, when the coiling temperature is less than 450 ° C., the generation rate of the internal oxide layer is slow, and the amount of internal oxidation becomes insufficient.

  Further, in the present invention, the temperature of the inner peripheral portion, the outer peripheral portion and the width direction end portion of the coil is made higher in advance than the other portions before winding the coil, so that the amount of internal oxidation is made uniform. Specifically, in cooling on the run-out table, cooling is performed in which the target values of the coiling temperature at the front end and tail end in the longitudinal direction of the hot-rolled steel sheet are changed stepwise as shown in FIG. (Hereinafter, such cooling is referred to as step cooling).

  As a result, as shown by the broken line in FIG. 4, it is possible to increase the temperature of the inner and outer peripheral parts where the cooling rate is fast at the initial stage of cooling, and to secure an internal oxide layer with an appropriate thickness. be able to.

  In the present invention, as the region to be higher than the temperature of the steady portion, in the present invention, the region corresponding to the length of 5 to 30% along the longitudinal direction of the front end portion and the tail end portion of the hot-rolled steel plate, the reason is as follows. Street.

  That is, in the region where the step cool region corresponds to a length of less than 5% along the longitudinal direction, the high temperature region is small and the heat is diffused in a short time by thermal diffusion, so that an internal oxide layer is generated. This time cannot be secured sufficiently. On the other hand, if the region where the step cool is performed exceeds the region corresponding to the length of 30% along the longitudinal direction, the temperature of the entire coil rises, and it is avoided that the amount of internal oxidation becomes excessive as a whole. Absent.

  For this reason, in this invention, the area | region which performs a step cool is made into the area | region corresponded to the length of 5 to 30% along the longitudinal direction of the front-end | tip part and tail-end part of a hot-rolled steel plate.

  About the temperature of this site | part, it is good to set it as the temperature 50-100 degreeC higher than a stationary part. This is because if the temperature is less than 50 ° C., the temperature is soaked in a short time due to thermal diffusion, so that a sufficient time for generating the internal oxide layer cannot be secured. On the other hand, if it exceeds 100 ° C., the temperature of the entire coil rises and the amount of internal oxidation becomes excessive as a whole.

  For this reason, in this invention, about the area | region corresponded to the length of 5-30% along the longitudinal direction of the front-end | tip part and tail end part of a hot-rolled steel plate, it is preferable to make it 50-100 degreeC temperature higher than a stationary part. It is what.

  Next, in the present invention, when cooling water is supplied to the upper surface and lower surface of the hot-rolled steel sheet to cool the hot-rolled steel sheet, the cooling water is supplied from the width end of the hot-rolled steel sheet toward the center. The reason for this is that if the region where the cooling water supply is cut off is less than 50 mm, the region where the temperature is high is small, and the temperature is soaked quickly due to thermal diffusion, and the internal oxide layer is generated. This is because it cannot be secured.

  If the cooling water blocking area exceeds 150 mm, the coil temperature after winding rises and the internal oxidation amount becomes excessive as a whole. For this reason, in the present invention, when cooling water is supplied to the upper and lower surfaces of the hot-rolled steel sheet for cooling, the cooling water is supplied to the region of 50 to 150 mm from the width end of the hot-rolled steel sheet toward the center. It was decided to block.

  By blocking the cooling water, the region of 50 to 150 mm from the end in the width direction toward the center thereof can be made higher than the temperature of other parts as shown in FIG. As shown in FIG. 2, at the initial stage of starting cooling, the temperature at the end in the width direction where the cooling rate is fast can be maintained at a high temperature, and an appropriate amount of internal oxidation can be ensured.

  The cooling water is blocked using a bowl-shaped edge mask such as a shielding plate that covers the area.

  If the rise in temperature due to the application of the edge mask is less than 50 ° C., the temperature is soaked immediately by thermal diffusion, so the time for forming the internal oxide layer cannot be ensured. The amount of internal oxidation becomes excessive.

  For this reason, in this invention, the temperature of the 50-100 mm area | region in the width | variety edge part of a hot-rolled steel plate becomes like this. Preferably it makes 50-100 degreeC higher than the temperature of a stationary part.

で求めることができる。 The generation rate of the inner oxide layer can be expressed by an Arrhenius type equation, and the thickness (δ) of the inner oxide layer is

Can be obtained.

The thickness (δ) of the internal oxide layer was clarified that the internal oxide layer formed after winding the hot-rolled steel sheet was expressed by the above equation (1) as a result of various experiments and research. ^Yes , the plating property is improved when the thickness (δ) (m) of the internal oxide layer at room temperature after winding satisfies the condition of 1.0 × 10 ⁻⁶ ≦ δ ≦ 4.0 × 10 ⁻⁶ Is done.

When the thickness (δ) of the internal oxide layer is less than 1.0 × 10 ⁻⁶ m (1.0 μm), the Si-deficient layer is thin, and Si oxide is generated on the steel sheet surface during annealing. It is inevitable that the plating property deteriorates.

On the other hand, if the thickness (δ) of the internal oxide layer exceeds 4.0 × 10 ⁻⁶ m (4 μm), the internal oxide layer of Si cannot be removed by the pickling treatment, and the plating property deteriorates. Unavoidable.

Therefore, in the present invention, the thickness (δ) of the internal oxide layer is preferably in the range of 1.0 × 10 ⁻⁶ ≦ δ ≦ 4.0 × 10 ⁻⁶ .

  The thickness (δ) of the internal oxide layer can be adjusted by controlling the time te until the coil temperature reaches room temperature.

  Specific examples of the hot-rolled steel sheet that can be used in the present invention include low-carbon steel containing about 1 mass% of Si and about 2 mass% of Mn (containing about 0.1 mass% of C).

  A hot-rolled steel sheet (steel type: TS590 MPa class high tensile steel) having a thickness (finished sheet thickness) of 3.0 mm, a width of 1000 mm, and a length of 650 m hot-rolled by applying the equipment shown in FIG. After introducing into the cooling equipment installed on the exit side of the inter-finishing mill and cooling under the conditions shown in Table 1 (applying at least one of step cool and edge mask), the steel plates to be cooled are listed in Table 1. Was wound into a coil at the temperature shown in FIG.

  And then, the steel sheet wound in a coil shape is rewound, pickled, cold-rolled, annealed, and further plated, and the quality status (yield) of the obtained plated steel sheet (as a product) The ratio (%) of the whole portion that satisfies the above criteria was investigated. The results are shown in Table 1 together with the results of Comparative Examples 1 to 3. The cooling conditions after the winding, the pickling conditions, the annealing conditions, and the plating can be performed under the following conditions.

Cooling conditions after winding: Cooling in the coil yard.
Pickling treatment conditions: Same as normal operation (type of acid solution: hydrochloric acid, concentration: 5 to 10%, conveyance speed: 150 to 300 mpm).
Annealing treatment conditions: heating at an average heating rate of 8 ° C./s or higher up to a temperature range of 700 ° C. or higher, holding 15 to 600 s at a temperature range of 800 to 900 ° C., cooling, and 10 to 200 s at a temperature range of 450 to 550 ° C. Retention.
Plating treatment conditions: Hot-dip galvanizing treatment is performed in an Al-containing bath at 460 ° C., the amount of adhesion is controlled to 40 g / m ² per side by gas wiping, and then alloyed at 500 ° C.

  Comparative Example 1 is a case of cooling to 400 ° C. without applying either a step cool or an edge mask in cooling in a cooling facility, and Comparative Example 2 is a step cool of an edge mask. In the case of winding at 650 ° C. without applying any of them, Comparative Example 3 is the case of winding at 550 ° C. without applying any of step cool and edge mask.

  On the other hand, Invention Example 1 applies a step cool and cools the region of 20% at both the front end portion and the tail end portion of the hot-rolled steel sheet so as to be 10 ° C. higher than the steady portion (without applying an edge mask) 550. In the case of winding at ℃, Invention Example 2 applies Step Cool as in Invention Example 1 so that the region of 20% at the tip and tail ends of the hot-rolled steel sheet is 60 ° C. higher than the steady portion. It is a case where it cools (it does not apply an edge mask) and it winds up at the temperature of 550 degreeC, and the invention example 3 arrange | positions the edge mask which becomes a width of 100 mm over the full length of a cooling equipment, and is 100 mm wide edge part of a hot-rolled steel plate. When cooling the region of 10 mm higher than the steady portion (without applying step cool) and winding it at a temperature of 550 ° C., the invention example 4 has an edge mask with a width of 100 mm over the entire length of the cooling equipment. Place hot rolled steel sheet When the region having a width of 100 mm is cooled to 60 ° C. higher than the steady portion (no step cool is applied) and wound at 550 ° C., Invention Example 5 further applies a step cool to hot rolling. An edge mask having a width of 100 mm is arranged over the entire length of the cooling equipment so that a 20% region of both the front end and the tail end of the steel plate is 60 ° C. higher than the steady portion, and the width end of the hot rolled steel plate is 100 mm. This region is a case where the region is cooled so as to be 60 ° C. higher than the steady portion and wound at 550 ° C.

  In Comparative Example 1, the coiling temperature was low, the plating property was poor, and the yield was 30%. In Comparative Example 2, since the coiling temperature was high and the plating property was poor, the yield was 30% as in Comparative Example 1.

  About Comparative Example 3, although the coiling temperature is within the range of 450 to 600 ° C., the plating performance is poor at the inner peripheral portion of the coil, the portion corresponding to the outer peripheral portion, and the end portion in the plate width direction, and the yield. Remained at 60%.

  On the other hand, the invention example 1 did not apply the edge mask, so the plating property at the end in the width direction was poor, but the hot rolled steel sheet corresponding to the inner and outer peripheral portions of the coil by applying step cool There was a tendency for the plating properties of the tip and tail ends to improve, and the yield was 65%.

  Inventive Example 2 did not apply the edge mask, and although the plating performance was poor at the end in the width direction, the application of Step Cool applied the hot rolled steel sheet corresponding to the inner and outer peripheral portions of the coil. The tip and tail end plating properties were good, and the yield was 75%.

  Inventive Example 3 did not apply step cool, so the plating properties of the tip and tail ends of the hot-rolled steel sheet were poor, but by applying an edge mask, the temperature at the end in the plate width direction was compared with the steady portion. Since the temperature was increased by 10 ° C., the plating property tended to be improved, and the yield was 65%.

  In addition, in the invention example 4, the step cool was not applied, so the plating properties at the front end portion and the tail end portion of the hot-rolled steel sheet were bad, but the plating property at the end portion in the plate width direction was good due to the application of the edge mask. The yield was 75%.

  In invention example 5, by applying step cool and edge mask, the plating properties of the front end portion, tail end portion, and plate width direction end portion of the hot-rolled steel sheet corresponding to the inner peripheral portion and outer peripheral portion of the coil are good. The yield was confirmed to improve to 98%.

  ADVANTAGE OF THE INVENTION According to this invention, the hot rolled steel plate suitable for manufacturing the original plate of a hot dip galvanized steel plate can be supplied stably.

DESCRIPTION OF SYMBOLS 1 Heating furnace 2 Rough rolling mill 3 Continuous hot finishing rolling mill 4 Cooling device 5 Winding machine

Claims (4)

In a method in which a hot rolled steel sheet having a Si content of 0.2 mass% or more is introduced into a cooling facility disposed downstream of the hot rolling line, and cooling is performed by supplying cooling water to the upper and lower surfaces of the steel sheet.
At least one of a region corresponding to a length of 50 to 150 mm from the width end of the hot-rolled steel plate toward the center thereof, and a length of 5 to 30% along the longitudinal direction of the tip and tail ends of the hot-rolled steel plate. The method for cooling a hot-rolled steel sheet is characterized by maintaining a temperature higher than the temperature of the stationary part.   The method for cooling a hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet has a winding target temperature of 450 to 600 ° C.   The region corresponding to the length of 50 to 150 mm from the width end of the hot-rolled steel sheet toward the central portion and the length of 5 to 30% along the longitudinal direction of the front end portion and the rear end portion of the hot-rolled steel plate is the steady portion. The method for cooling a hot-rolled steel sheet according to claim 1 or 2, wherein the temperature is higher by 50 to 100 ° C. The hot-rolled steel sheet according to any one of claims 1 to 3, wherein the thickness (δ) of the internal oxide layer of Si at room temperature after winding satisfies the following condition: Cooling method.
Record
1.0 × 10 ⁻⁶ ≦ δ ≦ 4.0 × 10 ⁻⁶ (1)
here,

t: Time (seconds)
tc: Winding start time (seconds)
te: Time to room temperature (seconds)
T: Absolute temperature (K)
a: 8-9 × 10 ⁻⁶
b: 1.5-2.5 × 10 ⁴

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